Rates of cancer and cardiovascular disease have shown very little improvement over the past two decades, and the incidence of Type II diabetes mellitus is increasing at an alarming rate. Recent reports estimate that approximately 30% of total cancer deaths are related to poor exercise and nutrition, and other reports have suggested that, when taking into consideration both cardiovascular disease and cancer, inactivity contributes to as many as 250,000 premature deaths per year (Booth et al., 2002). Despite the benefit of regular physical activity in the prevention of cancer and other debilitating illnesses, 75% of the U.S. population do not get the recommended amount of physical activity as defined by 30 minutes of moderate intensity physical activity 5 or more days per week (CDC, 2001), and 40% of the population is completely sedentary (USDHHS, 1996). The objective of the proposed research is to understand the mediators and moderators of a well tested, individually tailored, print-based intervention to increase exercise behavior among sedentary adults. Using a randomized, controlled intervention trial, the proposed study will address three primary and one secondary hypotheses: 1) A previously tested and validated exercise promotion intervention (c.f., Marcus et al., 1998) is successful at helping sedentary individuals initiate and maintain a moderate intensity physical activity regimen, as compared to a health and wellness control intervention, 2) Increases in positive attitudes, perceived normative support, self-efficacy, and intentions to exercise will mediate the effectiveness of the intervention, 3) That increased positive mood, and better temperature, stress, and lactate regulation immediately after exercise challenge (assessed in the laboratory) will moderate the effectiveness of the intervention, and 4) Secondarily, we will test whether gender, race/ethnicity, and two recently suggested genetic factors (BDNF and OPRM1) moderate the effectiveness of the intervention. The rigorous assessment of how and for whom an exercise promotion intervention is effective will provide information for future development of intervention strategies and content, as well as allow the targeting of exercise content to individuals for whom it is most likely to be effective.